An image forming apparatus is known which conveys a sheet-shaped recording medium (hereinafter collectively referred to as ‘sheet’) while forming an image on the sheet. The image forming apparatus comprises an image forming unit for forming a visible image (toner image) with a toner and a fixer for fixing the toner image on a sheet with heat and pressure. The image forming apparatus comprises a paper discharging tray for overlapping and stacking a plurality of sheets on which a toner image is fixed. However, if the temperature or the stack pressure of the plurality of sheets stacked on the paper discharging tray is increased, then the state of the toner is changed, which may result in the poor quality of images.